mm: Convert workingset to XArray

We construct an XA_STATE and use it to delete the node with
xas_store() rather than adding a special function for this unique
use case.  Includes a test that simulates this usage for the
test suite.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

include/linux/swap.h

@@ -306,15 +306,6 @@ void workingset_update_node(struct xa_node *node);
 		xas_set_update(xas, workingset_update_node);		\
 } while (0)
 
-/* Returns workingset_update_node() if the mapping has shadow entries. */
-#define workingset_lookup_update(mapping)				\
-({									\
-	radix_tree_update_node_t __helper = workingset_update_node;	\
-	if (dax_mapping(mapping) || shmem_mapping(mapping))		\
-		__helper = NULL;					\
-	__helper;							\
-})
-
 /* linux/mm/page_alloc.c */
 extern unsigned long totalram_pages;
 extern unsigned long totalreserve_pages;